Slats are devices on the leading edge of an aircraft wing which are deployed during takeoff and landing to increase the lift of the wing. During landing it is desirable for the slats to be fully deployed for maximum lift. Also, during landing it is desirable to open up a small slot between the slat and the wing fixed leading edge allowing a small amount of high pressure air from the lower surface to reach the upper surface, where it helps postpone the stall. However, during takeoff whilst it is desirable to deploy the slats at least partially to increase lift, it is preferable that there is no slot between the slat and the wing because this increases drag, noise and fuel consumption.
A so called “sealed slat” seals against the wing fixed leading edge when in its retracted (cruise) configuration. The slat is typically carried on slat tracks at either end of the slat so that the trailing edge of the slat closely follows the “D-nose” profile of the wing fixed leading edge. However, in practice, due to the wing span-wise curvature from the in-flight lift forces produced by the wing, a small gap exists in the takeoff configuration between the trailing edge of the slat and the D-nose profile. This is because, at takeoff, the slats are only constrained by the slat tracks at either end of the slat, and not in the middle of the slat. This gap can produce several hundred kilograms of drag overall in the takeoff condition for a commercial airliner.
FIG. 1 illustrates the sealing problems of a sealed slat configuration at takeoff. FIG. 1 is a front view of an aircraft wing 1 at takeoff looking aft. The wing 1 includes a root portion 2 and a tip portion 3. During flight, aerodynamic loads on the wing 1 cause the tip portion 3 to be deflected upwardly and this deflection is shown exaggerated in FIG. 1. The wing 1 includes a fixed aerofoil portion 4 having a fixed leading edge D-nose that runs the full span of the wing. The wing 1 has deployable slats 5 (shown transparent in FIG. 1) mounted to the leading edge of the fixed aerofoil portion 4. The slats 5 are movable between a retracted position in which they are flush with the aerodynamic leading edge of the fixed aerofoil portion 4, and an extended position in which they are deployed forwardly and downwardly so as to open up a slot between the slat 5 and the fixed aerofoil portion 4. The slats 5 are each mounted upon a pair of slat tracks 6 (note only one pair of slat tracks are shown in FIG. 1) which are mounted on rollers in the leading edge of the fixed wing portion 4. The slat tracks 6 are movable within the rollers under the control of the slat actuator mechanism, i.e. slat drive shaft, (not shown) of conventional type.
In the cruise condition, with the slats 5 retracted, the slats 5 are each held down by the slat tracks 6 at each end, and a “hold-down rib” (not shown) on the fixed leading edge at the middle of the slat. However, when the slats 5 are deployed, the slats are only constrained by the slat tracks 6 (i.e. the middle of the slats are no longer held down). Whilst the ends of the slats 5 are constrained by the slat tracks 6, the curvature of the wing 1 due to the aerodynamic forces causes gaps g1 and g2 to be opened up between the trailing edge of the slats 5 and the leading edge of the fixed wing portion 4 in the middle of the slats 5. This gap can reduce aerodynamic performance and lead to increased drag which is undesirable when the wing 1 is in its high lift configuration with the slats 5 deployed. The gaps g1 and g2 may be several millimeters.
One solution would be to adapt the profile of the D-nose such that the sealing between the trailing edge of the slats and the D-nose is improved. Where the span-wise wing curvature is small, profiling the D-nose may be an adequate solution to this sealing problem. However, on larger wing curvatures, especially at the mid and outboard leading edge of the wing, profiling the D-nose may cause the slats to scrape against the D-nose profile as they are retracted.